The Rambler( W.A.)

I have two (32 and40l) Engels mounted in enclosed gull wing boxes on camper trailer.There are two scuffer type vents on each box at the top which are open while fridges in use.This has worked but am thinking of installing a fan in each box to improve ventilationin hot conditions.Any ideas on where to put fans and whether they should be blowing air out or sucking air in to the boxes.I would prefer to use the existing vents as is.I have also added insulation to the gull wing boxes.

If the Engels both have internal fans to blow air across the condensers in each one, I would have the additional fans to draw air into the boxes and direct the airflow to the fan area of each Engel. That way you will get the slight air pressurization which will be expelled through the exit vent, but importantly, the air will actually pass where it is needed, to cool the condensers.
I use a Waeco and a 39 Engel in a vehicle and direct fan assisted air from a window vent to the fans in both fridges. Without it they run much more and use more electricity/ AH and simply swelter in their own exhaust heat.

.
Yes, I agree, on the inlet. As RMD says, some pressurisation will be of benefit. Also, the fan will be passing cooler air than at the exit so less stress on the fan.
It is not a 'big-deal' however, so if it is more convenient for mounting, place it on the exit.
You could also consider using a simple thermostat to control the fan and so reduce power consumption.

If the fans you use are 100mm dia computer fans, they can be wired in series and so run at lower speed but still move air, you don't need high flow amp dragging fans, all you need is to change the air and have it take away the elevated temp.

With one fan in each box delivering air to each box, the current draw is quite low, about 2.5 AH to 3 AH every 24 hours so can be left running all the time. No need for thermo switches etc. That small amount of current use is far less than the additional power two fridges will consume while continually trying to get rid of their own heat which is being reabsorbed in through their walls.
You have to stop/reduce the stewing in the box condition.

A lot of people say they use fridges in back of vehicles and canopies and boxes and are OK, but their fridge HAS to be running far more than they need to be purely because of the heat soak factor.
If they are running 24/7 then they use vast amounts of power and usually the frozen is, just frozen, nowhere near -18C. Perhaps they are oblivious to what is really happening.

Unless you have return air it will be a bit pointless. Most gull wing boxes have breathers with filter paper. Mine does. For the heat load of two fridges I’d think one breather mounted high and another down low would be needed. A fan in either would help although mounted in the top one facing out has a couple of advantages. Mobs like Aussie swag have their fans on temperature cutout circuits as well.
I run a single fridge, no fan. It goes ok. Paint colour has a lot to do with internal temps as well. A coat of ceramic reflective paint would work well if sun is an issue.

Some sort of Ram Air (snorkel) while moving and a marine style bilge blower into the boxes when stopped? Ram air would push ambient temp air into the box for venting as currently set up. Pretty easy to unclip a ram tube and install (push fit) a bilge blower